1. Field of the Invention
The invention relates to a method and apparatus for forming and displaying rainbow holograms. In particular, the invention provides a hologram arrangement which can be displayed by a light source contained within the hologram display unit.
2. Discussion of the Background
Holograms are widely used for a variety of purposes including information displays and/or amusement devices. Holograms advantageously provide a three-dimensional display, with the image being displayed including the appearance of depth.
One type of hologram is referred to as a rainbow hologram. A rainbow hologram is typically a white light viewable hologram which is conventionally formed as shown in FIG. 1. As schematically shown in FIG. 1, a previously formed original hologram, often referred to as H1, is utilized for forming a holographic image on a second holographic plate, often referred to as H2. In forming the second hologram H2, the light source 20 illuminates the original or master hologram H1, often via mirrors 60, 62, 64 and a beam splitter 61, such that the light source also provides a reference beam directly upon the second holographic plate H2. As shown, H1 is typically disposed at approximately a 45.degree. angle with respect to the direction of the illumination light, with H2 parallel to H1. However, in forming the rainbow hologram, only a slit of the original hologram H1 is allowed to illuminate the second holographic plate H2 for each illumination. Confinement of the illumination to a slit is achieved either by utilizing a slit beam produced, for example, by a cylindrical lens, or by masking the first holographic plate H1 with a mask M having a slit aperture, with the arrow A representing light emanating from the slit in the mask M. As a result, illumination of the second holographic plate H2 not only reconstructs the image formed by illuminating H1, but also reconstructs the slit in space.
As shown in FIG. 2, when this type of hologram is reconstructed using white light 56' to illuminate the second holographic plate H2, a plurality of reconstructed images are formed, for example as shown at 65, 66, 67 corresponding to red, green and blue reconstructed images. Each of the reconstructed images will have a slit S therein such that a viewer must view the recorded object from an angle/position in which all three of the slits are aligned. If viewed from a position in which the slits are not aligned, the image will either not be visible, or may only be partially visible. Typically, a holographic display requires an external light source to reconstruct or illuminate the image. Such arrangements usually consume excessive amounts of space, and require precise positioning of the light source with respect to the hologram being displayed. This can severely restrict the applications for which a holographic display can be utilized, and when utilized, the display is often inconvenient.
Accordingly, a holographic display system is desired which can allow for the display of a holographic image, while minimizing the space requirements for the display. The display with such a system should be able to conveniently and reliably display one or more holographic images. Preferably, such a display should also be capable of providing a high quality full or pseudo-color holographic image, without requiring a great deal of space and/or coordination in mounting an external light source to illuminate the display.